The invention relates to two fields that can be broadly categorized as "image reading" and "image writing." Its primary intended application in the image reading field would be as a tandem scanning confocal microscope, although it could also potentially be used for other applications, for example as a high-resolution document scanner, or as a reader for optical mass storage media, etc. The invention's primary intended application for image writing would be as a microlithography printer for semiconductor manufacture; however this field may also include applications such as document printing, photographic reproduction, etc. The following description will focus on the confocal microscopy and microlithography applications, although the specification can be applied by obvious extension to other applications as well.
A confocal microscope (Ref. 1) is similar to a conventional microscope except that the illumination is filtered by a small pinhole which is focused to a diffraction-limited microspot on the sample, and (in the case of a reflection confocal microscope) the light reflected from the sample is again filtered by the same pinhole. The focused beam is raster-scanned across the sample (by scanning either the pinhole or the sample) to build up a high-resolution raster image of the sample. (A transmission confocal microscope is similar, except that separate pinholes are used to filter the illumination and transmitted light.) In comparison to conventional microscopes a confocal microscope has superior lateral image resolution and also exhibits extremely fine depth resolution.
A tandem scanning confocal microscope of the Nipkow type (see Ref. 1, Chap. 14) uses an array of pinholes, rather than a single pinhole, to achieve a very high image frame rate. The pinholes are formed on a disk which spins at a high rate to provide real-time imaging. A drawback of the Nipkow-type system is that its field size is limited by the performance of conventional microscope objectives. Given the field size limitations of commercial high-power objectives it would take a very long time for a Nipkow-type system to scan, for example, a complete semiconductor wafer, even with its high image frame rate.
In comparison to typical microscopy applications, field size requirements for microlithography steppers are far more demanding. Current steppers must achieve high-resolution, flat-field, and low-distortion imaging performance comparable to high-quality microscope objectives, but over a field size of around 20 mm or greater. This level of performance is attained by using massive, multielement, all-glass projection lenses or catadioptric systems such as the Perkin-Elmer Micralign and Wynne-Dyson systems (Ref. 2, Chap. 8). The optics in such systems must be manufactured to submicron accuracies, and submicron alignment and dimensional stability tolerances must be held over large distances between massive optical and mechanical components to maintain resolution, focus and overlay accuracy. The technical difficulties associated with the combined requirements for high image resolution and large field size pose significant challenges to the further advancement of optical microlithography for semiconductor applications.